O295and o296 sponsorship start date validation

https://osraav.atlassian.net/browse/OSRA-295
https://osraav.atlassian.net/browse/OSRA-296

Model validation for Sponsorship.start_date 
- date can't be blank
- date can't be an arbitrary string
- date can't be later than the first of next month

plus rspec  and cucumber tests

Needs ruby 2.1.5
Me and Andrey paired on all this.
But I prefer to keep the date stubbed. Model testing are unit tests and one of the rules of unit tests are that it should be tested in isolation, it should not depend on outside factors like Date. 
This was in the SaaS course too.